An academic LOR should establish how you know the student, how you rank them among peers you have taught, and one concrete example of their work. Committees weight the ranking claim heavily — 'top 5% of students I have supervised' carries far more than 'excellent student'.
Someone who supervised your work directly and can give specific examples. A junior lecturer who taught you closely writes a far stronger letter than a department head who barely knows you.
One page, three to four paragraphs. Longer letters dilute the specific evidence that makes them persuasive.
It happens, and some recommenders ask for a draft. The recommender must review, edit and sign it in their own voice — the letter has to be genuinely theirs.